SYNOPSIS Carbachol and magnesium sulphate were used to increase the motility of the intestine in healthy volunteers. The effect was estimated by recording changes in abdominal sounds. Serum 5-hydroxytryptamine levels were assayed and found to be higher after gastrointestinal stimulation.5-Hydroxytryptamine (5-HT) is probably produced in the argentaffin cells in the intestine. Before the identification of the substance enteramine with 5-HT, Erspamer had shown its presence in extracts of the alimentary tract, and Feldberg and Toh (1953) found that the distribution of 5-HT corresponded to that of the argentaffin cells.A crucial experiment to demonstrate the origin of 5-HT was performed by Toh (1954). Using the technique of stomach perfusion devised by Dale and Feldberg (1934), he showed that there was a higher concentration of 5-HT in the portal blood of dogs than in the blood at the periphery. Similar experiments have recently been performed in patients by means of a cardiac catheter (Erspamer and Testini, 1959). They demonstrated more 5-HT in the hepatic vein than in the blood of the inferior vena cava. Haverback and Davidson (1958) reported extremely low levels of blood 5-HT in a patient with extensive resection of the large and small bowel, and Rosenberg, Davis, Moran, and Zimmermann (1959) showed that resection of the gastrointestinal tract in dogs reduced the serum 5-HT. The half-life of radioactive 5-HT in the bowel is shorter than in the platelets (Udenfriend and Weissbach, 1958), suggesting that platelets obtain 5-HT from the alimentary tract.

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Shortly after isoniazid (I.N.A.H.) was first used in the treatment of tuberculosis, it became apparent that side-effects were not uncommon. Thompson (1952) lists euphoria, hyperreflexia, vertigo, muscular twitching, insomnia, paraesthesiae and transient loss of memory among the reactions observed. These were noted to be mild and reversible but did suggest involvement of the nervous system.